CN112460837A - Titanium dioxide selective wave absorber based on DBS algorithm and design method - Google Patents

Titanium dioxide selective wave absorber based on DBS algorithm and design method Download PDF

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CN112460837A
CN112460837A CN202011433061.9A CN202011433061A CN112460837A CN 112460837 A CN112460837 A CN 112460837A CN 202011433061 A CN202011433061 A CN 202011433061A CN 112460837 A CN112460837 A CN 112460837A
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姜鑫鹏
杨俊波
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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Abstract

The invention is applied to the technical field of passive radiation cooling, and particularly relates to a titanium dioxide selective wave absorber based on a DBS algorithm and a design method thereof, so as to achieve the purpose of thermal radiation cooling. The wave absorber is composed of a periodic structure and comprises an algorithm optimization enhancement area, a medium absorption layer and a silver reflection layer which are sequentially arranged from top to bottom; the side length P of one period unit is more than or equal to 5 mu m and less than or equal to 15 mu m; the invention has the following advantages: 1. the selective wave absorber realizes the nearly perfect absorption of electromagnetic waves in the atmospheric infrared window range; 2. compared with the traditional radiation cooling structure, the material provided by the invention is simpler and thinner; 3. the processing technology adopted by the invention is to punch holes on the upper layer of titanium dioxide material, and transfer the titanium dioxide after punching to the silver substrate or generate the lower layer of silver reflecting surface by using a film coating mode, thereby being simpler in manufacturing technology and beneficial to batch production.

Description

Titanium dioxide selective wave absorber based on DBS algorithm and design method
Technical Field
The invention is applied to the technical field of passive radiation cooling, and particularly relates to a titanium dioxide selective wave absorber based on a DBS algorithm and a design method thereof, so as to achieve the purpose of thermal radiation cooling.
Background
With the continuous exploration of the nature and the development of science and technology, passive radiative cooling is systematically researched as an emerging technology. The principle is that the temperature is reduced by absorbing the heat on the earth surface and radiating the energy to the outer space by utilizing the electromagnetic wave in the wavelength range of the atmospheric transparent window. At present, most of energy sources are used for realizing cooling, and the research of the passive radiation cooling technology is expected to improve the current situation, so that the cooling can be realized under the condition of no energy source and no power consumption. On the other hand, researches prove that the radiation cooling technology can be applied to solving the problem of land desertification, and the radiation cooling technology is utilized to generate internal and external temperature difference to further collect condensed water, so that the survival rate of plant planting in desert is increased.
Studies have shown that the absorption of a material for a particular wavelength is equivalent to the emissivity for that particular wavelength. The radiation cooling technology requires high emissivity of the device for the atmospheric window wavelength range, namely, a selective absorber for the window wavelength range is sought. Conventional radiation-cooled selective absorbers can be broadly divided into two categories, one of which is the use of titanium oxide, zirconium oxide, silicon and silver and a top layer periodic gate design to perform the radiation cooling function. However, the selective wave absorber made of the multilayer material has great difference between theoretical results and experimental results due to the mutual influence among the multiple layers. In addition, the multiple layers of material clearly increase the thickness of the material. Another selective absorber performs the function of radiation cooling by using new polymer materials and some chemical means to make some materials with chemically doped particles or pore structures. There is a great uncertainty in this design method, which is closely related to the manufacturing process. In addition, the structure selective absorption range produced by chemical means and novel polymers is much larger than the transparent window range. This can cause the selective wave absorber to heat up and affect the cooling efficiency of the selective wave absorber.
With the wide research of the algorithm, a plurality of wave absorber structures with smaller thickness and improved performance are generated. Of course, most wave absorbers based on algorithm research still realize selective absorption based on the multilayer diffraction generated by the refractive index change between layers and mutual coupling. The principle is still based on multilayer different materials, the difference is that the layer thicknesses of the different materials are optimized, and the design concept is still limited by the traditional multilayer design concept. In addition, due to the difference of optimization conditions of the algorithm, the performance of the selective wave absorber of some algorithm designs is inferior to that of the traditional selective wave absorber. In recent years, some DBS algorithm-based architectures have exhibited superior performance over traditional designs. For example, the on-chip integrated mode multiplexing device designed by Yingjie Liu et al based on DBS algorithm has the characteristics of excellent performance, simple material, manufacturable structure and the like, and solves the cross-talk problem existing for a long time in the traditional design integrated circuit design, namely "Yingjie Liu, Ke Xu, Shuai Wang, Weihong Shen, Hucheng Xie, Yujie Wang, Shumin Xiao, Yong Yao, Jiangbing Du, Zuyuan He and Qinghai Song, arbitraryral route-division multiplexed circuits for integration. Nature Communications (2019)". Therefore, the device designed based on the DBS algorithm has a very wide application prospect.
Disclosure of Invention
The invention aims to solve the technical problems of mutual influence of multiple layers of materials of the traditional radiation cooling wave absorber and poor device radiation cooling effect caused by optimizing the selective wave absorber by utilizing an algorithm, and provides a selective wave absorber based on a DBS algorithm, which is applied to the field of radiation cooling.
The technical scheme adopted by the invention is as follows: a titanium dioxide selective wave absorber based on DBS algorithm is composed of a periodic structure, and comprises an algorithm optimization enhancement area 1, a medium absorption layer 2 and a silver reflection layer 3 which are sequentially arranged from top to bottom; the side length P of one period unit is more than or equal to 5 mu m and less than or equal to 15 mu m;
the thickness t of the algorithm optimization enhancement area 1 is within the range of 0.5 mu m to 3 mu m; the thickness w of the medium absorption layer 2 is within the range of 5 mu m and not more than w and not more than 25 mu m, and the thickness h of the silver reflection layer 3 is within the range of h and not more than 0.5 mu m; the side length a of a small square cell in each period unit satisfies a ═ P/N, wherein P is the period unit side length, and N is the number of the cells equally divided by the side length of one period unit.
The algorithm optimization enhancement area 1 and the medium absorption layer 2 both adopt titanium dioxide materials.
The invention also provides a design method of the selective wave absorber, which comprises the following specific steps:
step 1: determining materials and dimensions according to design requirements; the whole structure is defined to be composed of an upper titanium dioxide layer and a lower silver reflecting layer, wherein the thickness L of the upper titanium dioxide layer is t + w, t is the thickness of the algorithm optimization enhancement area 1, w is the thickness of the medium absorption layer 2, the range of L is 5.5 mu m or more and L or less and 28 mu m or less, and the range of the thickness h of the lower silver reflecting layer is h or more and 0.5 mu m or less.
Calculating by adopting FDTD simulation software to obtain parameters related to the initial structure and the wavelength: reflectivity R0(λ), transmittance T0(λ)；
Calculating the absorption A of the initial structure0(lambda) and emissivity ε0(λ) wherein ε0(λ)＝A0(λ)＝1-R0(λ)-T0(λ)；
Step 2: defining an initial design domain of the DBS algorithm optimization unit structure:
the upper layer titanium dioxide is divided into an algorithm optimization enhancement layer 1 and a medium absorption layer 2, wherein the initial design domain refers to the algorithm optimization enhancement layer 1 periodic unit structure. Each periodic unit is a square area with the side length of P, and the distance between every two adjacent units is 0; dispersing each unit into an NxN square cell, wherein the side length a of each square cell is P/N, and the design variables are the number of the square cells and the state of the square cells: with or without holes;
wherein: p is more than or equal to 5 mu m and less than or equal to 15 mu m, and N is more than or equal to 1 and less than or equal to 10;
and step 3: optimization model for DBS algorithm design
The optimization model of DBS algorithm design is as follows:
Figure BDA0002819082520000021
wherein EgMean absorption, x, in the range of the atmospheric transparent window representing the current structurei,jExpressing as material attribute, punching or not, i and j respectively correspond to the row and column of the corresponding cell, i, j is less than or equal to N; wherein the table is shown as 1 without punchingThe puncturing is represented by 0 and lambda1Represents the initial wavelength of the incident electromagnetic wave, and is set to be 8 μm; lambda [ alpha ]nThe cutoff wavelength for the incident electromagnetic wave is set to 13 μm, which includes the entire wavelength range of the atmospheric transparent window.
And 4, step 4: optimizing the design domain by utilizing an optimization model designed by the DBS algorithm; establishing a corresponding initial optimization model by adopting FDTD simulation software, and analyzing the reflectivity R of the corresponding wavelength obtained by the corresponding structureg(lambda) and transmittance Tg(lambda) obtaining emissivity epsilon corresponding to the wavelengthg(λ)：εg(λ)＝Ag(λ)＝1-Rg(λ)-Tg(lambda) and deducing the average emissivity E of the corresponding window rangeg. For a given number of unit cells, taking the material value of the unit cell as a design variable and taking the average emissivity EgThe maximum is an optimization target, and a direct binary search algorithm (DBS) is utilized to carry out topology optimization design based on material distribution, so that the selective wave absorber unit configuration meeting the requirements is obtained.
The specific optimization process is as follows:
given the initial value of each cell, the initial value of the variable is set to x as above1,1＝x1,2＝x1,3＝······＝xN,NSequentially scanning each unit cell of the optimized selective absorption area, changing the state of the scanned unit cell, and calculating the current EgThe value of (c). Comparing the average emissivity value of the current corresponding window range with the average emissivity value of the corresponding window range when the cell state is not changed, if the average emissivity value of the current corresponding window range is improved, keeping the new state of the scanning cell, otherwise, restoring the cell to the original state; and continues to scan for the next cell state. The scanning order in this process is to scan column by column starting with the first cell (first row and first column) and to iterate multiple times. Wherein the defined columns are arranged from left to right in the horizontal direction and the rows are arranged from top to bottom in the vertical direction. The judgment basis for stopping the optimization of the DBS algorithm is that after all the unit cells in the selective absorption area are scanned in one round, the comparison is carried out after all the unit cells are scanned in the previous roundAnd (5) as a result of the objective function, terminating optimization when the change value of the average emissivity of the two objective functions corresponding to the window range is lower than 0.1%, and outputting an optimal cell state, thereby obtaining the optimized titanium dioxide selective wave absorber cell structure.
Compared with the prior art, the invention has the following advantages:
1. due to the material characteristics of the silver and titanium dioxide materials and optimization through an algorithm, the selective wave absorber realizes the nearly perfect absorption of electromagnetic waves in the range of an atmospheric infrared window (8-13 mu m).
2. The selective wave absorber of the invention only uses two materials, titanium dioxide and silver, and the two materials are very easy to obtain, and are simpler and thinner than the materials proposed by the traditional radiation cooling structure.
3. Compared with a complex processing technology of a multilayer material, the processing technology adopted by the invention is to punch holes on the upper layer of titanium dioxide material and transfer the punched titanium dioxide to the silver substrate or generate the lower layer of silver reflecting surface by using a film coating mode, so that the manufacturing technology is simpler and is beneficial to batch production.
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FIG. 1 is a periodic structure diagram of a titanium dioxide selective wave absorber based on DBS algorithm;
FIG. 2 is a structure diagram of a periodic unit of a titanium dioxide selective wave absorber based on DBS algorithm;
FIG. 3 is a schematic diagram of a two-dimensional planar structure of a cell of the device of the present invention; (wherein the perforated part is black)
FIG. 4 is a schematic diagram of the initial structure of a device of the present invention (corresponding to FIG. 2) when one cell is not optimized;
FIG. 5 is an emission spectrum of 8-13 μm with and without a titanium dioxide structure;
Detailed Description
The invention will now be further described with reference to the following examples and drawings:
the specific embodiment is as follows: a titanium dioxide selective wave absorber applied to radiation cooling and a unit structure design of the metamaterial wave absorber are as follows:
(1) determining materials and dimensions according to design requirements; determining the structure of the double-layer metamaterial wave absorber, wherein the determined initial structure consists of titanium dioxide and a silver reflecting layer, and the figure is 3. The upper layer titanium dioxide is divided into an algorithm optimization enhancement layer 1 and a medium absorption layer 2, the thickness of the algorithm optimization enhancement layer 1 is 2 μm, the thickness of the medium absorption layer 2 is 12 μm, the thickness L of the upper layer titanium dioxide is t + w is 2+12 is 14 μm, and the thickness h of the lower layer silver reflection layer is 0.5 μm. The wavelength dependent parameters of the initial structure were calculated and the emissivity spectrum of the initial structure is shown in fig. 5 as a solid grey line.
Let λ be the wavelength of the incident wave, R0(λ) is the reflectance at the corresponding wavelength, T0(λ) is the transmittance at the corresponding wavelength, A0(lambda) is the absorptivity of the wave-absorbing structure to the corresponding wavelength, and is equivalent to the emissivity epsilon of the corresponding wavelength0(λ), can be represented as
ε0(λ)＝A0(λ)＝1-R0(λ)-T0(λ)
The lowest silver reflecting layer can be regarded as an ideal conductor in the infrared band, thereby ensuring that the transmissivity is 0, namely T0(lambda) is 0, so that the emissivity epsilon of the structure corresponding to the wavelength0(λ) can be represented as
ε0(λ)＝A0(λ)＝1-R0(λ)
Thereby obtaining the average emissivity E of the atmospheric window wave band under the initial structure0：
Figure BDA0002819082520000041
Wherein E0The average emissivity of the atmospheric window wave band of the initial structure, lambda1Denotes the initial wavelength, λnThe initial wavelength is 8 μm and the cutoff wavelength is 13 μm in the atmospheric window range.
The titanium dioxide material has good high emissivity for atmospheric window wave band, so the initial structure only has the titanium dioxide of which the upper layer is not optimized and the silver reflecting layer of which the lower layer is not optimizedAverage emissivity E of the segments086.5%. This average emissivity E0And also as an initial average emissivity in optimization of DBS algorithm to compare whether optimization is achieved with and without puncturing.
(2) Initial design domain defining the structure of the DBS algorithm unit: each design unit is a square area with the side length P being 10 μm, the design areas are respectively scattered into square cells of 10 × 10, the side length a of each cell being 10/10 being 1 μm, wherein the state of each cell is a design variable in the design model of the DBS algorithm.
In the present design, the design variables involved are the states of each cell. For a given configuration of the number of cells, the state value of each cell is air and titanium dioxide (the spatial distribution problem of the two materials), 0 represents the value of the design variable as air, and 1 represents the value of the design variable as titanium dioxide. Taking a 10 × 10 square cell as an example, the material distribution problem is converted into a problem that 100 small squares take a value of 0 or 1.
The upper-layer perforating structure optimized through the algorithm generates refractive index mutation (the difference between the refractive indexes of titanium dioxide and air) in the algorithm optimization enhancement area, and further generates a local mode, so that the emissivity of the electromagnetic wave to the atmospheric window waveband is improved.
(3) Optimization model for DBS algorithm design
Setting initial value of variable as x1,1＝x1,2＝x1,3＝······＝xN,NThe material property representing these squares is titanium dioxide. A direct binary search algorithm (DBS) is adopted for topology optimization, and the aim is to obtain a selective wave absorber configuration with the maximum average emissivity in an atmospheric transparent window wave band (8-13 mu m).
The optimization model of DBS algorithm design is as follows:
Figure BDA0002819082520000051
wherein xi,jExpressed as material property, perforated or not, i andj corresponds to the row and column of the corresponding cell, i, j is not greater than N, which is known as N being 10; where no puncture is denoted by 1 and a puncture is denoted by 0. Average emissivity E with an objective function of atmospheric window rangegThe optimization goal is to maximize its value. Rg(λ) is the reflectivity of the current structure at the corresponding wavelength, λ1Denotes the initial wavelength, λnThe initial wavelength is 8 μm and the cutoff wavelength is 13 μm in the atmospheric window range. Wherein x isi,jThe state of the cells in the ith row and the jth column is shown, the value of 0 shows that the material of the small square is air, and the value of 1 shows that the material of the small square is titanium dioxide.
(4) Optimizing the design domain by utilizing an optimization model designed by the DBS algorithm; establishing a corresponding initial optimization model by adopting FDTD simulation software, and analyzing the reflectivity R of the corresponding wavelength obtained by the corresponding structureg(lambda) and transmittance Tg(lambda) and the like. Solving for ε as mentioned earlier0(lambda) obtaining emissivity epsilon of current structure corresponding to wavelengthg(lambda) and deriving the average emissivity E over the atmospheric windowg. For a given number of unit cells, taking the material value of the unit cell as a design variable and taking the average emissivity EgThe maximum is an optimization target, and a direct binary search algorithm (DBS) is utilized to carry out topology optimization design based on material distribution, so that the selective wave absorber unit configuration meeting the requirements is obtained.
The specific optimization process is as follows:
given the initial value of each cell, the initial value of the variable is set to x as above1,1＝x1,2＝x1,3＝······＝xN,NSequentially scanning each unit cell of the optimized selective absorption area, changing the state of the scanned unit cell, and calculating the current EgThe value of (c). Comparing the average emissivity value of the current corresponding window range with the average emissivity value of the corresponding window range when the cell state is not changed, if the average emissivity value of the current corresponding window range is improved, keeping the new state of the scanning cell, otherwise, restoring the cell to the original state; and continue to scanOne cell state. The scanning order in this process is to scan column by column starting with the first cell (first row and first column) and to iterate multiple times. Wherein the defined columns are arranged from left to right in the horizontal direction and the rows are arranged from top to bottom in the vertical direction. The judgment basis for terminating the optimization of the DBS algorithm is that after scanning all cells in the selective absorption area in one round, the target function result after scanning all cells in the previous round is compared, the optimization is terminated when the change value of the average emissivity of the window range corresponding to the two target functions is lower than 0.1%, and the optimal cell state is output, so that the optimized titanium dioxide selective wave absorber unit structure is obtained.
Finally, it can be seen that for a given unit side length P of 10, dividing a unit into 10 × 10 cells has the largest average emissivity. The schematic diagram of the periodic structure and the unit structure is shown in fig. 1 and fig. 2. The designed titanium dioxide selective wave absorber is of a three-layer structure and comprises an algorithm optimization enhancement area 1, a medium absorption layer 2 and a silver reflection layer 3. The material of the algorithm optimization selection absorption region 1 is titanium dioxide crystals and air, and the thickness of the optimization selection absorption region 1 is t 2 mu m; the material of the absorption layer 2 is titanium dioxide, the thickness of the absorption layer 2 is 12 micrometers, the material of the reflection layer 3 is silver, and the thickness of the reflection layer is 0.5 micrometers. Fig. 4 shows all cell states in one cell after algorithm optimization, wherein a black cell represents a punch state, i.e., the above-mentioned state 0 (the cell filling state is air), and a white cell represents the state 1. As shown in fig. 5, the emissivity of the invented titanium dioxide selective absorber is shown compared to the emissivity in the initial state. The invented device can realize high absorption of 96.7% for the electromagnetic wave of 8-13 micron window band. Compared with the average emissivity of the window without algorithm optimization, the average emissivity of the device is greatly improved, and the performance of the device applied to radiation cooling is further improved.

Claims (3)

1. A titanium dioxide selective wave absorber based on DBS algorithm is characterized in that: the wave absorber is composed of a periodic structure and comprises an algorithm optimization enhancement area (1), a medium absorption layer (2) and a reflection layer (3) which are sequentially arranged from top to bottom; the side length P of one period unit is more than or equal to 5 mu m and less than or equal to 15 mu m;
the thickness t of the algorithm optimization enhancement region (1) is within the range of 0.5 mu m to 3 mu m; the thickness w of the medium absorption layer (2) is within the range of 5 mu m and not more than 25 mu m, and the thickness h of the silver reflection layer 3 is within the range of h and not more than 0.5 mu m; the side length a of a small square cell in each period unit meets the condition that a is equal to P/N, wherein P is the side length of the period unit, and N is the number of the cells equally divided by the side length of the period unit; the algorithm optimization enhancement area (1) and the medium absorption layer (2) are made of titanium dioxide materials, and the reflecting layer (3) is made of silver.
2. A titanium dioxide selective wave absorber based on DBS algorithm according to claim 1, wherein: the wave absorber parameters are as follows: p is 10 μm, N is 10, a is 1 μm, t is 2 μm, w is 12 μm, and h is 0.5 μm.
3. A method for designing the selective wave absorber according to claim 1 or 2, characterized by comprising the following steps:
step 1: determining materials and dimensions according to design requirements; defining the whole structure to be composed of an upper titanium dioxide layer and a lower silver reflecting layer, wherein the thickness L of the upper titanium dioxide layer is t + w, t is the thickness of the algorithm optimization enhancement region (1), w is the thickness of the medium absorption layer (2), the range of L is more than or equal to 5.5 mu m and less than or equal to 28 mu m, and the range of the thickness h of the lower silver reflecting layer is more than or equal to 0.5 mu m;
calculating by adopting FDTD simulation software to obtain parameters related to the initial structure and the wavelength: reflectivity R0(λ), transmittance T0(λ)；
Calculating the absorption A of the initial structure0(lambda) and emissivity ε0(λ) wherein ε0(λ)＝A0(λ)＝1-R0(λ)-T0(λ)；
Step 2: defining an initial design domain of the DBS algorithm optimization unit structure:
the upper layer titanium dioxide is divided into an algorithm optimization enhancement layer (1) and a medium absorption layer (2), wherein the initial design domain refers to a periodic unit structure of the algorithm optimization enhancement layer (1): each periodic unit is a square area with the side length of P, and the distance between every two adjacent units is 0; dispersing each unit into an NxN square cell, wherein the side length a of each square cell is P/N, and the design variables are the number of the square cells and the state of the square cells: with or without holes;
wherein: p is more than or equal to 5 mu m and less than or equal to 15 mu m, and N is more than or equal to 1 and less than or equal to 10;
and step 3: optimization model for DBS algorithm design
The optimization model of DBS algorithm design is as follows:
Figure FDA0002819082510000011
wherein EgMean absorption, x, in the range of the atmospheric transparent window representing the current structurei,jExpressing as material attribute, punching or not, i and j respectively correspond to the row and column of the corresponding cell, i, j is less than or equal to N; where no puncture is denoted by 1, puncture is denoted by 0, and λ1Represents the initial wavelength of the incident electromagnetic wave, and is set to be 8 μm; lambda [ alpha ]nA cutoff wavelength for an incident electromagnetic wave set to 13 μm, which includes a wavelength range of the entire atmosphere transparent window;
and 4, step 4: optimizing the design domain by utilizing an optimization model designed by the DBS algorithm; establishing a corresponding initial optimization model by adopting FDTD simulation software, and analyzing the reflectivity R of the corresponding wavelength obtained by the corresponding structureg(lambda) and transmittance Tg(lambda) obtaining emissivity epsilon corresponding to the wavelengthg(λ)：εg(λ)＝Ag(λ)＝1-Rg(λ)-Tg(lambda) and deducing the average emissivity E of the corresponding window rangeg(ii) a For a given number of unit cells, taking the material value of the unit cell as a design variable and taking the average emissivity EgThe maximum is an optimization target, and a direct binary search algorithm is utilized to carry out topology optimization design based on material distribution, so that a selective wave absorber unit configuration meeting requirements is obtained;
the specific optimization process is as follows:
given the initial value of each cell, the initial value of the variable is set to x as above1,1＝x1,2＝x1,3＝······＝xN,NSequentially scanning each unit cell of the optimized selective absorption area, changing the state of the scanned unit cell, and calculating the current EgA value of (d); comparing the average emissivity value of the current corresponding window range with the average emissivity value of the corresponding window range when the cell state is not changed, if the average emissivity value of the current corresponding window range is improved, keeping the new state of the scanning cell, otherwise, restoring the cell to the original state; and continuing to scan the next cell state; the scanning sequence in the process is that the first unit cell is scanned in columns and iterated for a plurality of times; wherein the defined columns are arranged from left to right in the horizontal direction and the rows are arranged from top to bottom in the vertical direction; the judgment basis for terminating the optimization of the DBS algorithm is that after scanning all cells in the selective absorption area in one round, the target function result after scanning all cells in the previous round is compared, the optimization is terminated when the change value of the average emissivity of the window range corresponding to the two target functions is lower than 0.1%, and the optimal cell state is output, so that the optimized titanium dioxide selective wave absorber unit structure is obtained.
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CN113063240A (en) * 2021-03-19 2021-07-02 大连理工大学 Composite structure surface in field of radiation-enhanced refrigeration

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